Resistive Switching Phenomena Research Articles

Implementation of 8-bit reservoir computing through volatile ZrOx-based memristor as a physical reservoir

In this study, we employed a sputtering process to construct a memristive device within the ITO/ZrOx/TaN structure for implementing neuromorphic computing. Initially, we scanned the basic electrical properties of the ITO/ZrOx/TaN device using a DC voltage sweep on the top ITO electrode. A highly uniform gradual resistive switching phenomenon was observed over 100 cycles. The current decay in the low-resistance state was effectively controlled by the volatile memory properties. Gradual conductance changes for potentiation and depression were achieved by applying electrical pulses, enabling the establishment of multi-level conductance states. In addition, the emulation of various synaptic functions was achieved by following the learning rules of SRDP, EPSC, STDP, ADSP, Pavlovian associative learning, and PPF. Finally, 8-bit reservoir computing was demonstrated in cost-effective pattern generation and recognition, highlighting the ITO/ZrOx/TaN device’s advantageous memory storage properties for synaptic characteristics.

Theoretico-experimental analysis of bistability in the oscillatory response of a TaOx ReRAM to pulse train stimuli

Fading memory is the capability of a physical system to approach a unique asymptotic behaviour, irrespective of the initial conditions, when stimulated by an input from a certain class. Standard stimuli from the AC periodic class typically induce fading memory effects in non-volatile memristors, as uncovered for the first time back in 2016. Very recently, a deep investigation of resistance switching phenomena in a TaOx resistive random access memory cell revealed the capability of the nano-device to exhibit one of two possible oscillatory behaviours, depending upon the initial condition, when subject to a particular periodic excitation. This interesting finding was, however, left unexplained. Bistability is the simplest form of local fading memory. In a system, endowed with local fading memory under a given stimulus, the initial condition does not affect the long-term behaviour of the state as long as it is drawn from the basin of attraction of either of the distinct coexisting state-space attractors (two limit cycles for the periodically forced memristor acting as a bistable oscillator). Here, the history of the system, encoded in the initial condition, is, thus, erasable only locally through ad hoc stimulation. Motivated by the discovery of local history erase effects in our resistive random access memory cell, this study applies a powerful system-theoretic tool, enabling the analysis of the response of first-order systems to square pulse train-based periodic stimuli, known as the time-average state dynamic route, to an accurate physics-based mathematical model, earlier fitted to the nano-device, to determine a strategy for specifying the parameters of an excitation signal, consisting of the sequence of two square pulses of opposite polarity per period so as to induce various forms of monostability or multistability in the non-volatile memristor. In particular, as an absolute novelty in the literature, experimental measurements validate the theoretical prediction on the capability of the device to operate as one of two distinct oscillators, depending upon the initial condition, under a specific pulse train excitation signal. The coexistence of multiple oscillatory operating modes in the periodically forced resistive random access memory cell, an example par excellence of their unique non-linear dynamics, may inspire the development and circuit implementation of novel sensing and mem-computing paradigms.

Modified Donor End Caps for Binary-to-Ternary WORM Memory Conversion in N-Heteroaromatic Systems.

A series of novel D-π-A type organic small molecules have been designed, synthesized, and demonstrated for non-volatile resistive switching WORM memory application. The electron-deficient phenazine and quinoxaline units were coupled with various functionalized triphenylamine end caps to explore the structure-property correlations. The photophysical investigations displayed considerable intramolecular charge transfer, and the electrochemical analysis revealed an optimum band gap of 2.44 to 2.83 eV. These factors and the thin film morphological studies suggest the feasibility of the compounds as better resistive memory devices. All the compounds indicated potent non-volatile resistive switching memory capabilities with ON/OFF ratios ranging from 103 to 104, and the lowest threshold voltage recorded stands at -0.74 V. A longer retention time of 103 s marks the substantial stability of the devices. The phenazine-based compounds outperformed the others in terms of memory performance. Exceptionally, the compound with -CHO substituted triphenylamine exhibited ternary memory performance owing to its multiple traps. The resistive switching mechanism for the devices was validated using density functional theory calculations, which revealed that the integrated effect of charge transfer and charge trapping contributes significantly to the resistive switching phenomena.

Building Uniformly Structured Polymer Memristors via a 2D Conjugation Strategy for Neuromorphic Computing.

Polymer memristors represent a highly promising avenue for the advancement of next-generation computing systems. However, the intrinsic structural heterogeneity characteristic of most polymers often results in organic polymer memristors displaying erratic resistive switching phenomena, which in turn lead to diminished production yields and compromised reliability. In this study, a 2D conjugated polymer, named PBDTT-BPQTPA, is synthesized by integrating the coplanar bis(thiophene)-4,8-dihydrobenzo[1,2-b:4,5-b]dithiophene (BDTT) as an electron-donating unit with a quinoxaline derivative serving as an electron-accepting unit. The incorporation of triphenylamine groups at the quinoxaline termini significantly enhances the polymer's conjugation and planarity, thereby facilitating more efficient charge transport. The fabricated polymer memristor with the structure of Al/PBDTT-BPQTPA/ITO exhibits typical non-volatile resistive switching behavior under high voltage conditions, along with history-dependent memristive properties at lower voltages. The unique memristive behavior of the device enables the simulation of synaptic enhancement/inhibition, learning algorithms, and memory operations. Additionally, the memristor demonstrates its capability for executing logical operations and handling decimal calculations. This study offers a promising and innovative approach for the development of artificial neuromorphic computing systems.

Transient N-GQDs/PVA nanocomposite thin film for memristor application

In recent years quantum dot (QDs) based resistive switching devices(memristors) have gained a lot of attention. Here we report the resistive switching behavior of nitrogen-doped graphene quantum dots/Polyvinyl alcohol (N-GQDs/PVA) degradable nanocomposite thin film with different weight percentages (wt.%) of N-GQDs. The memristor device was fabricated by a simple spin coating technique. It was found that 1 wt% N-GQDs/PVA device shows a prominent resistive switching phenomenon with good cyclic stability, high on/off ratio of ~102 and retention time of ∼104 s. From a detailed experimental study of band structure, we conclude that memristive behavior originates from the space charge controlled conduction (SCLC) mechanism. Further transient property of built memristive device was studied. Within three minutes of being submerged in distilled water, the fabricated memory device was destroyed. This phenomenon facilitates the usage of fabricated memristor devices to develop memory devices for military and security purposes.

Long‐ and Short‐Term Memory Characteristics Controlled by Electrical and Optical Stimulations in InZnO‐Based Synaptic Device for Reservoir Computing

AbstractIn this study, the resistive switching phenomenon and synaptic mimicry characteristics of an indium tin oxide (ITO)/indium zinc oxide (IZO)/Al2O3/TaN device are characterized. The insertion of a thin Al2O3 layer via atomic layer deposition improves the resistive switching characteristics such as cycle‐to‐cycle and device‐to‐device uniformity and reduces the power consumption of the proposed device with respect to a single‐layer ITO/IZO/TaN device. The proposed device exhibits the coexistence of volatile and nonvolatile characteristics under optical and electrical measurement conditions. Nonvolatile memory characteristics with stable retention results are used for synaptic applications by emulating potentiation, depression, and spike‐timing‐dependent plasticity. Furthermore, the device shows volatile characteristics under ultraviolet‐light illumination, emulating paired‐pulse facilitation and excitatory post‐synaptic current responses. Finally, optical‐enhanced reservoir computing is implemented based on the nonlinear and volatile nature of the IZO‐based resistive random‐access memory device.

Stacked structure dependence on resistive switching characteristics in sumanene molecular memory

Nonvolatile memories using molecule (molecule memories) are attracting attention. This is because these materials are suitable for miniaturization and higher capacity of memories in terms of their properties and dimensions. We have already demonstrated that the metal–insulator–metal (MIM) devices with sumanene-inserted bilayer graphene show huge resistive switching characteristics. However, the reason why resistive switching occurs in the graphene/sumanene/graphene structure has yet to be clarified. In this work, to investigate the mechanisms of the resistive switching phenomenon in sumanene-inserted bilayer graphene, plural kinds of stacked MIM structures are fabricated and evaluated. As a result, the measurement results clearly show that the graphene/sumanene/graphene structure is indispensable in the resistive switching phenomenon. Furthermore, based on the temperature dependence of the resistive switching, it is confirmed that a significant I ON/I OFF ratio can be obtained at higher operation temperatures.

Implementation of Artificial Synapse Using IGZO-Based Resistive Switching Device.

In this study, we present the resistive switching characteristics and the emulation of a biological synapse using the ITO/IGZO/TaN device. The device demonstrates efficient energy consumption, featuring low current resistive switching with minimal set and reset voltages. Furthermore, we establish that the device exhibits typical bipolar resistive switching with the coexistence of non-volatile and volatile memory properties by controlling the compliance during resistive switching phenomena. Utilizing the IGZO-based RRAM device with an appropriate pulse scheme, we emulate a biological synapse based on its electrical properties. Our assessments include potentiation and depression, a pattern recognition system based on neural networks, paired-pulse facilitation, excitatory post-synaptic current, and spike-amplitude dependent plasticity. These assessments confirm the device's effective emulation of a biological synapse, incorporating both volatile and non-volatile functions. Furthermore, through spike-rate dependent plasticity and spike-timing dependent plasticity of the Hebbian learning rules, high-order synapse imitation was done.

Resistive Switching Effect in Ag‐poly(ethylene Glycol) Nanofluids: Novel Avenue Toward Neuromorphic Materials

AbstractConventional computation techniques face challenges of deviations in Moore's law and the high‐power consumption of data‐centric computation tasks. Neuromorphic engineering attempts to overcome these issues by taking inspiration from neuron assemblies, ranging from distributed synaptic plasticity through orchestration of oscillator‐like action potential toward avalanche dynamics. Although solid networks of nanoparticles (NPs) are proven to replicate fingerprints of criticality and brain‐like dynamics, the aspect of dynamic spatial reconfigurations in the connectivity of networks remains unexplored. In this work, Ag/poly(ethylene glycol) (PEG) nanofluids are demonstrated as potential systems to mimic the spatio‐temporal reconfiguration of network connections. The nanofluids are prepared by directly loading Ag NPs from the gas aggregation cluster source into liquid PEG. The NPs exhibit a negative zeta potential in PEG; if the potential difference is applied between two electrodes submerged in this nanofluid, the NPs migrate toward the anode, accumulate in its vicinity, and form a conductive path. Spikes of electric current passing through the path are detected, accompanied by resistive switching phenomena, similar to the random switching dynamics in solid NPs networks. The unique behavior of Ag/PEG nanofluids makes them promising for the realization of spatio‐temporal reconfigurations in network topologies with the potential to transition to 3D.

Implementation of Physical Reservoir Computing in a TaOx/FTO-Based Memristor Device

As one of the solutions to overcome the current problems of computing systems, a resistive switching device, the TiN/TaOx/fluorine-doped tin oxide (FTO) stacked device, was fabricated to investigate its capability in embodying neuromorphic computing. The device showed good uniformity during the resistive switching phenomenon under time and cycle-to-cycle dependent switching, which may be due to the oxygen reservoir characteristics of the FTO bottom electrode, storing oxygen ions during resistive switching and enhancing the device property. Based on the uniform switching phenomenon of the TiN/TaOx/FTO device, the pulse applications were performed to seek its ability to mimic the biological brain. It was revealed that the volatile and non-volatile nature of the device can be altered by controlling the pulse stimuli, where strong stimuli result in long-term memory while weak stimuli result in short-term memory. To further investigate the key functions of the biological brain, various learning rules such as paired-pulse facilitation, excitatory postsynaptic current, potentiation and depression, spike-rate dependent plasticity, and spike-time dependent plasticity were tested, with reservoir computing implemented based on the volatile characteristic of the TiN/TaOx/FTO device.

Resistive switching characteristics of MnO2-based thin film for transparent non-volatile ReRAM

The present study covers the fabrication of indium tin oxide (ITO)/manganese dioxide (MnO2)/ITO capacitor-based transparent resistive random access memory (ReRAM) device. The ITO/MnO2/ITO device exhibits 80 % transparency in the visible region along with steady bipolar resistive switching characteristics. The conduction mechanism investigation reveals that the current in low resistance state (LRS) is governed by Ohmic conduction. However, the dominance of trap-controlled space charge limited conduction (SCLC) mechanism in high resistance state (HRS) was observed. The resistive switching phenomenon in ITO/MnO2/ITO device was caused due to the formation and rupture of conduction filament via electron trapping and de-trapping in oxide-related traps (oxygen vacancies). In terms of reliability, the MnO2-based device exhibits good endurance up to 104 cycles and long retention for 104 s at room-temperature as well as at 85 °C. Cumulative probability distribution shows resistance ratio of 103 between LRS and HRS, which is sufficient to read memory states. These outcomes suggest that the ITO/MnO2/ITO-based transparent ReRAM could be a futuristic see-through electronic device for artificial intelligence (AI) application.

Nanoscale imaging of He-ion irradiation effects on amorphous TaOx toward electroforming-free neuromorphic functions

Resistive switching in thin films has been widely studied in a broad range of materials. Yet, the mechanisms behind electroresistive switching have been persistently difficult to decipher and control, in part due to their non-equilibrium nature. Here, we demonstrate new experimental approaches that can probe resistive switching phenomena, utilizing amorphous TaOx as a model material system. Specifically, we applied scanning microwave impedance microscopy and cathodoluminescence (CL) microscopy as direct probes of conductance and electronic structure, respectively. These methods provide direct evidence of the electronic state of TaOx despite its amorphous nature. For example, CL identifies characteristic impurity levels in TaOx, in agreement with first principles calculations. We applied these methods to investigate He-ion-beam irradiation as a path to activate conductivity of materials and enable electroforming-free control over resistive switching. However, we find that even though He-ions begin to modify the nature of bonds even at the lowest doses, the films' conductive properties exhibit remarkable stability with large displacement damage and they are driven to metallic states only at the limit of structural decomposition. Finally, we show that electroforming in a nanoscale junction can be carried out with a dissipated power of <20 nW, a much smaller value compared to earlier studies and one that minimizes irreversible structural modifications of the films. The multimodal approach described here provides a new framework toward the theory/experiment guided design and optimization of electroresistive materials.

Highly uniform resistive switching characteristics of KGM resistive memory through air plasma technology

In this study, a highly stable resistive switching behavior was obtained by utilizing konjac glucomannan, a biomaterial, as a dielectric layer through a solution-based process, and the ITO bottom electrode was modified by using an air plasma technique. Electrical results showed that the pretreatment device did not observe significant switching characteristics (∼101). The device, which underwent additional plasma treatment, was recognized for its significant improvements in this area, demonstrating a reduced set voltage (Vset = 0.37 V) with a 6% coefficient of variation, more than 200 consecutive cycles, and up to 90% electrical yield. Moreover, the treated device exhibited a dramatically higher memory window (>104) due to the relatively low off-state current. According to the XPS and UPS analysis, the work function increased from an untreated 4.3 to 5.79 eV as the treatment time increased to 180 s. The conductive filaments, which stemmed from oxygen vacancies, were introduced through plasma treatment to enhance the proportion of oxygen vacancies in the ITO films, thereby creating a stable and consistent filament path. Consequently, the systematic and reproducible resistive switching phenomenon was intensified. The reported results confirmed that the reliability and uniformity in bioelectronic devices can be accomplished through a simple and effective plasma technique. This approach paved the way for alternative applications of these devices.

Resistive switching like-behavior in FD-SOI Ω-gate transistors

In this work, the Resistive switching (RS) phenomenon is experimentally investigated in N-type FDSOI Ω-gate NW-FETs with high-k dielectric. The location along the channel of the conductive filament through the device dielectric during switching is analyzed. Finally, the effects of RS on the characteristic transistor curves are also presented.

Revelation of resistive switching mechanism in AlOx based RRAM device

Resistive random access memory (RRAM) device based on conductive filaments (CF) is a promising candidate for next-generation nonvolatile memory applications. A detailed understanding of the resistive switching phenomenon is very important for RRAM development. In this paper, a numerical physics-based drift/diffusion model is presented to examine the resistive switching characteristic and morphological evolution of CF channels in AlOx based RRAM device. The experiment phenomenon and simulated results suggest that CF channels formed in the oxide film are composed of Cu metal and oxygen vacancy.

Electrical properties and defect analysis of MAPbI3 thin films grown on TiO2 layer through a two-step drying process

We studied the electrical properties and defect states of methylammonium lead iodide perovskite (MAPbI3) thin film grown onto the TiO2 layer at a temperature of 120 °C via a two-step drying process. The TiO2 layer with thickness of 20 nm were formed on fluorine doped tin oxide/glass at 120 °C via atomic layer deposition. The MAPbI3 thin film showed α-phase structure with band gap energy of about 1.61 eV. For the MAPbI3/TiO2 with Ag top and bottom electrodes, the resistance switching phenomena were enhanced compared to the MAPbI3 film without the TiO2 layer. In the MAPbI3/TiO2, two deep level states of E1(Ec-0.38 eV) and H2 (Ev+0.82 eV) appeared from deep level transient spectroscopy measurements. The defect states of E1 and H2 are related to iodine interstitial (Ii) and iodine antisite (IPb), respectively. In the MAPbI3/TiO2, the defect of Ii appeared as the ionized states of Ii+ or Ii−, so that it was found that the defect state of Ii caused the MAPbI3/TiO2 to have the resistance random access memory property by the migration of the ionized states.

Monolayer MoS2 as a sensitive probe: Exploring the resistive switching mechanism of MoS2/NSTO heterostructures

Revealing the mechanism of the interface-type resistive switching remains challenging due to the difficulties in direct observation of the buried interface. As a transparent conductive electrode, monolayer MoS2 is suitable for studying resistance switching mechanisms directly at the interface due to its atomic-sized thickness and electron transfer sensitivity. In this study, a memory device based on the MoS2/Nb:SrTiO3 heterojunction was constructed. An apparent hysteresis loop can be observed, along with resistive switching phenomenon between high resistance state and low resistance state. In addition, the surface potential of MoS2 also varies significantly as the resistance state changes. Furthermore, the evolution of PL spectrum in MoS2 under electric fields presents a shift with the resistance state. Finally, using the sensitive response of the MoS2 monolayer, interface charge trapping/detrapping mechanism in MoS2/Nb:SrTiO3 heterojunction was confirmed.

Tunable Resistive Switching Behaviors and Mechanism of the W/ZnO/ITO Memory Cell.

A facile sol-gel spin coating method has been proposed for the synthesis of spin-coated ZnO nanofilms on ITO substrates. The as-prepared ZnO-nanofilm-based W/ZnO/ITO memory cell showed forming-free and tunable nonvolatile multilevel resistive switching behaviors with a high resistance ratio of about two orders of magnitude, which can be maintained for over 103 s and without evident deterioration. The tunable nonvolatile multilevel resistive switching phenomena were achieved by modulating the different set voltages of the W/ZnO/ITO memory cell. In addition, the tunable nonvolatile resistive switching behaviors of the ZnO-nanofilm-based W/ZnO/ITO memory cell can be interpreted by the partial formation and rupture of conductive nanofilaments modified by the oxygen vacancies. This work demonstrates that the ZnO-nanofilm-based W/ZnO/ITO memory cell may be a potential candidate for future high-density, nonvolatile, memory applications.

Controllable resistive switching behaviors in heteroepitaxial LaNiO3/Nb:SrTiO3 Schottky junctions through oxygen vacancies engineering

Perovskite oxide-based memristors have been extensively investigated for the application of non-volatile memories, and the oxygen vacancies associated with Schottky barrier changing are considered as the origin of the memristive behaviors. However, due to the difference of device fabrication progress, various resistive switching (RS) behaviors have been observed even in one device, deteriorating the stability and reproducibility of devices. Precisely controlling the oxygen vacancies distribution and shedding light on the behind physic mechanism of these RS behaviors, are highly desired to help improve the performance and stability of such Schottky junction-based memristors. In this work, the epitaxial LaNiO3 (LNO)/Nb:SrTiO3 (NSTO) is adopted to explore the influence of oxygen vacancy profiles on these abundant RS phenomena. It demonstrates that the migration of oxygen vacancy in LNO films plays a key role in memristive behaviors. When the effect of oxygen vacancies at the LNO/NSTO interface is negligible, improving the oxygen vacancies concentration in LNO film could facilitate resistance on/off ratio of HRS and LRS, and the corresponding conducting mechanisms attributes to the thermionic emission and tunneling-assisted thermionic emission, respectively. Moreover, it is found that reasonably increasing the oxygen vacancies at LNO/NSTO interface makes trap-assisted tunneling possible, also providing an effective way to improve the performance of the device. The results in this work have clearly elucidated the relationship between oxygen vacancy profile and RS behaviors, and give physical insights into the strategies for improving the device performance of Schottky junction-based memristors.

Controlled Switching of Bistable Nanophase Domains on a Silicon Surface.

Switching the crystalline phase of a material via electrostatic control is a proven strategy for developing memory devices such as memristors that are based on nonvolatile resistance switching phenomena. However, phase switching in atomic-scale systems is often difficult to control and poorly understood. Here, we explore nonvolatile switching of long 2.3 nm wide bistable nanophase domains in a Sn double-layer structure grown on Si(111), using a scanning tunneling microscope. We identified two mechanisms for this phase switching phenomenon. First, the electrical field across the tunnel gap continuously tunes the relative stability of the two phases and favors one over the other depending on the tunneling polarity. The second mechanism involves carrier injection into empty Sn orbitals. The coupling between these relatively long-lived hot electrons and surface phonons induces a lattice instability at sufficiently large tunneling current and provides access to a hidden metastable state of matter. This hidden state is nonvolatile but can be erased by choosing the appropriate tunneling conditions or raising the temperature. Similar mechanisms could possibly be exploited in phase-change memristor and field effect devices.